It is well known in the art that the strength of paper products can be increased by incorporating increased amounts of paper pulp into the product. However, this also increases the weight of the product at a financial cost. A preferred means of increasing the strength of the paper product is the addition of small amounts of additives that do not substantially increase the weight or cost of the paper.
It is also known in the art that addition of acrylamide/acrylic acid/methylene-bis-acrylamide terpolymers can improve the surface strength of paper, but this may lead to the loss of internal strength. Such terpolymers are described in published Japanese patent application 32-79491, where 0.1 to 10 parts by weight of a crosslinking agent having at least two double bonds is used. Water-swellable, crosslinked terpolymers of acrylamide, an alkali metal acrylate and a difunctional chemical crosslinker are also known, for example, as described in U.S. Pat. Nos. 3,247,171 and 4,525,527. These products are used, for example, as thickeners and sludge coagulants, and in sanitary goods.
There is still a need for an effective internal dry strength additive for paper that is water-soluble for convenience of operation and readily dilutable with water and that is easily miscible with the papermaking fibers.